The present invention relates to a system for transmitting first signals to, and receiving second signals from one or more remote transponders. More particularly, the invention relates to a radar system utilizing transponders which are capable of receiving an interrogating first signal, processing this signal and transmitting, in reply, a second signal that is derived from the first signal and contains encoded information.
Because the aforementioned encoded information normally includes an identification code which is unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as "labels". The entire system, including the interrogator/receiver apparatus and one or more passive transponders, is therefore often referred to as a "passive interrogator label system" or "PILS".
Passive interrogator label systems of the type to which the present invention relates are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 3,273,146 Horwitz, Jr. 3,706,094 Cole et al. 3,755,803 Cole et al. 3,981,011 Bell 4,058,217 Vaughan et al. 4,059,831 Epstein 4,263,595 Vogel ______________________________________
Such a system is also disclosed in the commonly-owned patent applications referred to above.
In general, a passive interrogator label system includes an "interrogator" for transmitting a first radio frequency signal; at least one passive transponder which receives this first signal, processes it and sends back a second radio frequency signal containing encoded information; and a receiver, normally located next to the interrogator, for receiving the second signal and decoding the transponder-encoded information.
The aforementioned patent application Ser. No. 509,523 discloses a passive interrogator label system in which the interrogator transmits a first signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, a frequency band that is freely available in many parts of the world for short range transmissions.
A passive (i.e., nonpowered) transponder associated with this system receives the first (interrogating) signal as an input and produces a second (reply) signal as an output. Passive signal transforming means within the transponder, which converts the first signal to the second signal, includes:
(1) A multiplicity of "signal conditioning elements" coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the first signal.
(2) A single "signal combining element" coupled to all of the signal conditioning elements for combining the intermediate signals (e.g., by addition or multiplication) to produce the second signal. This second signal is coupled out of the same or a separate antenna for transmission as a reply.
The signal conditioning elements and the signal combining element impart a known informational code to the second signal which identifies, and is associated with, the particular transponder.
The receiving and decoding apparatus associated with the system includes apparatus for receiving a second signal from the transponder and a mixer, arranged to receive both the first signal and the second signal, for performing four quadrant multiplication of these two signals. The mixer produces, as an output, a third signal containing the difference frequencies (or frequencies derived from the difference frequencies) of the first and second signals, respectively.
Finally, the system disclosed in the aforementioned U.S. patent application Ser. No. 509,523 includes a signal processor, responsive to the third signal produced by the mixer, for detecting the phases and amplitudes of the respective difference frequencies contained in the third signal, thereby to determine the informational code associated with the interrogated transponder.
This particular system has a number of advantages over passive interrogator label systems of the type disclosed in the issued U.S. patents referred to above. For example, this system exhibits substantially improved signal-to-noise performance over the prior known systems. Also, the output of the signal mixer--namely, the third signal which contains the difference frequencies of the first (interrogating) signal and the second (reply) signal--may be transmitted over inexpensive, shielded, twisted-pair wires because these frequencies are in the audio range. Furthermore, since the audio signal is not greatly attenuated when transmitted over long distances, the signal processor may be located at a position quite remote from the signal mixer.
In practice, the passive transponders used in the PILS described above comprise a microwave antenna and a surface acoustic wave ("SAW") device coupled to this antenna. Radiation, picked up by the antenna, is converted into electrical signals which are, in turn, converted into surface acoustic waves on the SAW device by one or more so-called "launch" transducers. These waves travel outward in opposite directions from opposite sides of these launch transducers and are then reconverted into electrical signals, after progressing along a plurality of paths of different lengths, either by separate receiving transducers or, after 180.degree. reflections, by the same (launch) transducers.
Transponders of this type are susceptible to interference which reduces the signal-to-noise ratio of the reply signal that is transmitted by the transponder antenna back to the interrogator. Such interference is primarily caused by reflections from the various metallized elements disposed on the surface of the SAW device in the aforementioned travel paths. Such elements include bus bars, delay pads and even the launch and receiving transducers themselves. The amplitudes of such reflections are directly proportional to the change in velocity, .DELTA.V, of a surface acoustic wave as it passes from a metallization-free surface area on the SAW device to a metallized surface area and vice versa. Such surface acoustic wave reflections are reconverted into electrical signals by the transducers in their paths of travel, resulting in spurious electrical signals that appear as noise in the transmitted reply signal.